1. Field of the Invention
The present invention relates to an etch-resistant inkjet ink and a method of manufacturing conductive patterns.
2. Description of the Related Art
Printed circuit boards are usually made by coating a photo resist layer on a copper sheet bonded to a non-conductive substrate, applying a temporary UV mask of a negative image of a desired conductive pattern, UV exposing the photo resist layer, removing the non-exposed photo resist layer by a developer, removing unwanted copper by etching, removing the exposed photo resist layer by an alkaline stripping bath, thereby leaving only the desired conductive copper pattern present on the non-conductive substrate.
Etching is the process of using a chemical, usually a strong acid or mordant, to cut into the unprotected parts of a metal surface. The use of developer to remove the photo resist layer, often 50 μm thick or more, results in extra cost and chemical waste. Therefore, it has been investigated if the developing step could be eliminated by UV curable inkjet printing an etch-resistant inkjet ink layer on the copper sheet, which after etching is removed in flakes by the alkaline stripping bath to expose the conductive copper pattern.
U.S. Pat. No. 5,270,368 (VIDEOJET) discloses a UV curable, etch-resistant ink for inkjet printing circuit boards comprising a resin formulation having at least two acrylate components, one of which is an aromatic acrylate having a pendant carboxyl group and one of which is an acrylated epoxy monomer or dimer, a photoinitiator and an organic carrier. The preferred organic carrier of methanol and methyl ethyl ketone is employed in a range of 40% to 90% by weight of the ink composition. These volatile organic solvents lead to latency problems of inkjet print heads making reliable inkjet printing in an industrial environment process problematic. Reducing the amount of organic solvent leads to a too high ink viscosity, because some aromatic acrylate compounds traditionally used for preparing photo resist coatings have very high viscosity. For example, the bisphenol A ethoxylated diacrylate (Photomer™ 4028) used in all the examples of U.S. Pat. No. 5,270,368 (VIDEOJET) has a viscosity of 800 to 1200 mPa·s at 25° C. These aromatic acrylate compounds are essential for having a good balance in adhesion so that the printed ink layer is etch resistant yet easily removable after etching, especially since many different etching conditions and etchants are used in industry.
WO 2004/106437 A (AVECIA) discloses a process for etching a metal or alloy surface which comprises applying an etch-resistant ink by inkjet printing to selected areas of the metal or alloy, exposing the etch-resistant ink to actinic radiation and/or particle beam radiation to effect polymerisation, optionally thermally treating the etch-resistant ink and then removing the exposed metal or alloy by a chemical etching process wherein the etch-resistant ink is substantially solvent-free. All the disclosed etch-resistant inks include an acidic polymerizable compound. The inclusion of acidic polymerizable compounds has some undesirable side effects such as increased viscosity and decreased ink stability and curing speed.
Another problem is the flake formation in the alkaline stripping bath. Stripping solutions are normally alkaline metal hydroxides, such as sodium or potassium hydroxide, or based on amines such as mono or tri ethanolamine and tetra methyl ammonium hydroxide. The stripping solution breaks the polymer chain at the cross-linking point of the three dimensional structure, which is formed during the polymerization of the resist and before the bond between the resist and the copper surface is broken. In order to extend the working lifetime of the stripping solution, it is necessary to filter the solution to remove the stripped flake of resist. If the flake size is too large it tends to adhere to stripping equipment disturbing the smooth running of the manufacturing process. If the flakes are too small they pass through the filter and return in the stripping bath. After a while these small flakes accumulate and also start to disturb the smooth running of the manufacturing process. These very small flakes tend to block the nozzles of the sprays of the stripping line. The stripped flake size depends mainly on the type of stripping solution, the concentration of the stripping solution, the temperature of the stripping solution and the design of the stripping equipment, etc. This multitude of influencing factors makes it very difficult to control the flake size to a desired size.
Hence, there remains a need for improved low viscous radiation curable inkjet inks suitable for reliable inkjet printing in an industrial etching process applicable to a wide range of etchants and etching conditions and exhibiting no problems with stripping and flake formation.